The present invention relates to a displacement pump, to a method of improving the operation thereof, and to the use of the displacement pump. The pump in accordance with the invention is particularly well suited for use in locations where relatively high pressure level performance is required of the pump and where the material to be pumped is problematic (e.g. the material being pumped may be particularly erosive or corrosive, or the heat resistance of the material may be poor). It is especially advantageous to use the displacement pump in accordance with the invention for pumping cellulosic fiber suspensions, synthetic fiber suspensions, and glass fiber suspensions in the pulp and paper industry and particularly in the manufacture of non-woven webs by the water-laid, or foam processes (e.g. see copending application Ser. No. 08/923,900 filed Sep. 4, 1997.
It is known from the prior art to pump cellulosic fiber suspensions, synthetic fiber suspensions, and glass fiber suspensions, either with a centrifugal pump or a displacement pump. The type of the pump is chosen primarily depending upon the consistency of the pulp slurry to be pumped, and/or the pressure level performance required of the pump. When, for instance, the pressure level performance required of a pump is so high that it cannot be reached with a centrifugal pump, a displacement pump--such as a rotary-lobe pump or a gear pump--is then normally used. Prior art pumps for this purpose are shown, for example, in the following U.S. Pat. Nos. 4,621,994; 4,913,629; 5,154,149; 5,318,425, and 5,567,140. These pumps normally operate well when they are used to pump clean cellulosic fiber suspensions, in which the fibers are soft and have a good heat resistance properties. Problems arise when pumping synthetic fibers or fibers coated with synthetics, e.g. with acrylic. One problem is gradual increasing consumption of power of the pump motor, and overheating of the pump. If measures are not taken to correct this, the shaft of the pump may break off or at least the safety clutch in the pump may be activated, so that the pump motor keeps on running but the rotor of the pump stands still.
The source of the above mentioned problems is the passage of fibers into the small clearance between the rotor and housing. In order to reach the required pressure level, the clearance between the rotor of the pump and the end surfaces of the pump casing must be kept small. In actual operation this causes the fibers to be pushed and squeezed in and ground between the rotatable rotor and the end surface of the casing. Consequently, the temperature of the fibers rises and synthetic fibers or fiber coatings with poor heat resistance start to melt and stick both to the end surface of the rotor and the interior of the casing. The clearance between the rotor and the casing then narrows further, and this leads to accelerated melting of new fibers or their coatings, and to further blocking of the clearance. In other words, the frictional force, which at first is practically non-existent between the rotor and the end surface of the casing, increases and this leads to increased consumption of power of the pump motor. When the clearance becomes narrower and narrower, the frictional force keeps growing to such an extent that the safety clutch of the pump will be activated, the shaft of the pump will break, or the pump motor will become overheated. In any case the process stops suddenly and time-consuming and expensive repair operations must be carried out.
Another problem typical to pumps of this type arises when the suspension to be pumped contains abrasive fibers, e.g. glass fibers. With glass fibers the problem is the abrasion of the rotor and the end surface of the casing. Because the fibers to be pumped are hard and heat resistant, they do not tend to melt in the clearance but rather cause the end surface of the rotor, or the end surface of the casing, or both, to be ground away, to accommodate the glass fibers. Thus the tightness of the pump is gradually reduced and consequently, the pressure level that can be obtained with the pump decreases substantially until the pump is no longer capable of producing the required pressure level, at which time the pump needs to be repaired.
The above problems, among other factors, exist in all displacement pumps, and particularly the most common pumps for this purpose, rotary-lobe pumps and gear pumps. Such pumps typically consist or comprise a casing and two rotors rotating in opposite directions in the casing. The vanes of the rotors operatively engage each other and thus seal the space between the casing of the pump and the rotor, and also the space between the rotors. There are typically two different types of constructions of these pumps. In the first type the rotors are mounted on bearings in the casing at both ends, e.g. as in U.S. Pat. No. 5,318,415, and in the second type only at the drive end, as in U.S. Pat. No. 4,621,994. Naturally, the construction with bearings at both ends is a more solid construction, but it is also more likely to be damaged because of the bearings at both ends and the sealing which is necessary for the bearings. For this reason pumps with bearings on only one end have gained in popularity.
The problems discussed above are at their worst at the free end of the rotors, where there is no natural flushing available to wash away the fibers from the clearance between the rotor head and the end of the casing of the pump, or to prevent the fibers from reaching the clearance. At the opposite end of the rotors as well as in pumps which have bearings at both ends of the rotors, there is a sealing element at both ends, usually of the stuffing box type, which prevents the fibers from reaching the bearings of the rotors. It is characteristic of pumps of this type that sealing water is directed to the space between the shaft and the sealing element. The sealing water lubricates the space between the bearing and the shaft and also washes away possible impurities. The sealing water is also an excellent flushing liquid for keeping the space between the end surface of the rotor and the end of the casing clean. This is why problems at the end of the rotor with bearings are rare. If they do arise, one may also expect that the seal itself will become damaged as a result of an inadequate flow of sealing water to the sealing element.
The purpose of the present invention is to resolve the above mentioned problems, so that it becomes possible to use a displacement pump to pump suspensions having poor heat resistant fibers, and suspensions having heat resistant but abrasive fibers. Also the invention seeks to increase the reliability of a displacement pump under these circumstances
According to one aspect of the present invention a displacement pump is provided, such as a rotary-lobe or gear pump, comprising the following components: A casing having an inlet and an outlet, and including a first end, and a second end remote from the first end, the second end having an interior surface. A plurality of rotors, connected to shafts, mounted within the casing, the shafts extending substantially perpendicularly to the interior surface of the second end of the casing. A bearing mounting each of the shafts adjacent the first end of the casing for rotation about an axis substantially perpendicular to the interior surface of the second end of the casing, and so that a clearance is defined between the rotors and the interior surface of the first end of the casing. A drive operatively connected to at least one of the shafts adjacent the first end of the casing. And, means for introducing flushing fluid into the clearance to keep the clearance substantially free of undesirable particles or fibers.
The means for introducing flushing fluid into the clearance to keep the clearance substantially free of undesirable particles or fibers may be any conventional structure for introducing a fluid flow into a clearance to keep the clearance clean, including conduits, pipes, nozzles, injectors, or the like. One particularly desirable means comprises a fluid-conducting opening extending through the second end of the casing at points at or adjacent a projection of each of the shafts to the second end. Alternatively, where at least one of the shafts has a fluid passageway therein, the means for introducing the flushing fluid may comprise a fluid passageway for transporting flushing fluid from adjacent the first end of the casing to the clearance at the second end of the casing. Alternatively the means for introducing flushing fluid into the casing may comprise a passage in the casing extending generally perpendicularly to the shaft axes of rotation, and at least one opening that opens from the passageway to the clearance. The flushing fluid may be any fluid that can perform the flushing function adequately without significantly adversely impacting on the slurry being pumped. Perhaps the two most desirable flushing fluids are sealing water and substantially fiber-free foam.
The clearance between the tops of the rotors and the inside surface of the casing second end is typically between 0.01-0.5 mm, e.g. between about 0.15-0.35 mm (or between about 0.01-0.25 mm, or between about 0.26-0.5 mm).
While bearings may be provided at both the first and second end of the casing, preferably the bearings mounting the shafts consist of only the bearings adjacent the first end of the casing, the shafts not engaging or mounted in the second end of the casing. If desired, at least one of the rotors may have vanes on the surface thereof adjacent the interior surface of the second end of the casing, for pumping fibers or particles radially outwardly from the clearance. The inlet and the outlet to the casing may be on opposite sides of the casing, with introducing or removing slurry being pumped generally transverse to the shaft axes of rotation.
According to another aspect of the present invention a system for handling the slurry of abrasive or poor heat resistant fibers or particles is provided. The system comprises: A displacement pump comprising: a casing having an inlet and an outlet, and including a first end, and a second end remote from the first end, the second end having an interior surface; at least one rotor, connected to a shaft, mounted within the casing, the shaft extending substantially perpendicularly to the interior surface of the second end of the casing; a bearing mounting the shaft adjacent the first end of the casing for rotation about an axis substantially perpendicular to the interior surface of the second end of the casing, and so that a clearance is defined between the rotor and the interior surface of the first end of the casing; a drive operatively connected to the at least one shaft adjacent the first end of the casing; and means for introducing flushing fluid into the clearance to keep the clearance substantially free of abrasive or poor heat resistance particles or fibers. A source of slurry having abrasive or poor heat resistance fibers or particles connected to the casing inlet. And, a source of flushing fluid operatively connected, including by a flushing fluid conduit, to the means for introducing flushing fluid.
The system further preferably comprises a check valve provided in the flushing fluid conduit to prevent fluid from passing out of the casing through the flushing fluid conduit. The at least one rotor and shaft may comprise two rotors and shafts. The clearance, and the means for introducing the flushing fluid, preferably are as described above. A flow meter and a control valve may be disposed in a conduit between the flushing fluid source and the check valve.
According to another aspect of the present invention a method of pumping a slurry of abrasive or poor heat resistant fibers (such as fibers having greater abrasive properties than cellulose fibers, and having poorer heat resistance than cellulosic fibers), using a displacement pump (such as described above) is provided. The method comprises the following steps: (a) Feeding a slurry of abrasive or poor heat resistance fibers having a consistency of between about 1-50% (e.g. 2-30%, or 5-20%) to the displacement pump. (b) Pumping the slurry with the displacement pump, by rotating the shaft about an axis of rotation so that the rotor contacts the slurry, to pressurize the slurry to a first pressure. And, (c) feeding flushing fluid into the clearance between the rotor and casing second end interior surface at a second pressure, greater than the first pressure, to substantially prevent abrasive or poor heat resistance fibers from building up or having undesirable effects on the rotor or casing adjacent the clearance.
Step (b) may be practiced to pump a fiber-foam slurry to a non-woven web former which produces a non-woven fibrous web, such as described in U.S. Pat. Nos. 3,716,449 and 3,871,952, for example. Alternatively the pump may pump a liquid slurry to a web former, or may otherwise be used in the pulp and paper art.
The method may comprise the further step of determining the amount of flushing fluid added to the clearance so that the affect of the flushing fluid on slurry consistency can be determined. This may be accomplished utilizing a flow meter disposed in a conduit feeding the flushing fluid into the clearance (pursuant to step (c)). Step (c) is typically practiced using sealing water or substantially fiber-free foam, although other suitable flushing fluids may be utilized.
The displacement pump may comprise at least two rotors, in which case step (d) is practiced to rotate both of the rotors at the same time while guiding the shafts with bearings only at the first end of the casing, the shafts and rotors being spaced from the second end of the casing. Step (c) may be practiced in any suitable manner that accomplishes the desired result of keeping the clearance substantially free of abrasive or poor heat resistance fibers or particles. For example step (c) may be practiced by introducing the flushing fluid through the casing second end interior surface in a direction substantially parallel to the axis of rotation. Alternatively where the shaft has a passageway therein extending from adjacent the first end of the casing to the clearance, step (c) is practiced by passing flushing fluid through the passageway from the first end of the casing to the clearance. Where the second end plate has a passageway therein substantially perpendicular to the axis of rotation of the rotor, step (c) may be practiced by feeding the fluid into the passageway, and then from the passageway into the clearance.
It is the primary object of the present invention to provide a longer life displacement pump, related system, and method of pumping a slurry, all designed for use with slurries having fibers or particles more abrasive and with poorer heat resistance than cellulose fibers. This and other objects of the invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.